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Using JSON and XML to save and restore parameters

Atle Borsholm

When writing IDL code, it is often convenient to be able to save and restore parameters used in the code. This could be algorithm constants or thresholds used to perform computations, or it might be the status of the checkboxes on a user interface. In earlier versions of IDL, a static structure was often used to store such information. In IDL 8.3, the ordered hash is probably the best choice for this purpose.

The ordered hash object is dynamic, and can be nested. Here is an example:

IDL> param = OrderedHash()

IDL> param['sigma'] = !dpi/4

IDL> param['xsize'] = 512

IDL> param['name'] = 'Exelis'

IDL> param


   "sigma": 0.78539816339744828,

   "xsize": 512,

   "name": "Exelis"


And with nesting:

IDL> tmp = OrderedHash('alpha', 1, 'beta', [255,240,190])

IDL> param['subparam'] = tmp

IDL> param


   "sigma": 0.78539816339744828,

   "xsize": 512,

   "name": "Exelis",

   "subparam": {

       "alpha": 1,

       "beta": [255, 240, 190]




The simplest way to save such data is to use IDL's SAVE procedure.  However, the resulting file will be in a binary format that is not accessible unless you use IDL's RESTORE procedure. If you want the configuration parameters to be editable in a text editor or accessible to another programming language, it is better to use a readable format such as JSON or XML. Here is an example using JSON:


IDL> json = JSON_SERIALIZE(param)

IDL> json



In this example "json" is a string representation of the OrderedHash object.



To restore the parameters at a later time, use:


IDL> newparam = JSON_PARSE(string(read_binary(filename)))


This will create a new ordered hash from the string that was read from a file in this case. The JSON format has a limited set of built-in types that it supports, so, IDL variables will not necessarily retain the exact same type after going through JSON_SERIALIZE and JSON_PARSE. For example, in this case, the array was converted into a list, since JSON arrays correspond more closely to IDL's list than an IDL array. Numbers are always returned as double precision floating point or long64 integers.


Finally, I am including a simple example of adding XML serialization and de-serialization to the ordered hash. Since OrderedHash inherits from Hash, the methods are added to Hash. The first method is hash__toxml.pro

function hash::ToXml,_ref_extra=extra

 compile_opt idl2,logical_predicate

 nl = string(10b)

 formats = hash(4,'(g0.9)',5,'(g0.17)',6,'("(",g0.9,",",g0.9,")")',9,'("(",g0.17,",",g0.17,")")')

 oXml = IDLffXmlDomDocument()

 oTop = oXml.CreateElement('TOP')

 parent = oXml.AppendChild(oTop)

 keys = self->Keys()

 foreach k, keys do begin

   val = self[k]

   type = size(val, /type)

   node = oXml.CreateElement(k)

   node.SetAttribute, 'TYPE', strtrim(type, 2)

   if isa(val, /array) then node.SetAttribute,'ARRAY', $

     strjoin(string(size(val, /dimension),format='(i0)'), ' ')

   format = formats.HasKey(type) ? formats[type] : '(i0)'

   str = (type eq 7)?strjoin(val, nl):strjoin(string(val, format=format), ' ')

   txt = oXml.CreateTextNode(str)

   _ = node.AppendChild(txt)

   _ = parent.AppendChild(oXml.CreateTextNode(nl))

   _ = parent.AppendChild(node)


 _ = parent.AppendChild(oXml.CreateTextNode(nl))

 if n_elements(extra) then oXml.Save, _strict_extra=extra

 return, oXml


With this method an ordered hash can be turned into XML using the following code:

IDL> param = OrderedHash()

IDL> param['sigma'] = !dpi/4

IDL> param['xsize'] = 512

IDL> param['name'] = 'Exelis'

IDL> param['theta'] = [[23, 334, 4.0, 41.0], [1.0, 2, 3, 4]]

IDL> !null = param.ToXml(string=str)

% Loaded DLM: XML.

% Compiled module: HASH::TOXML.

IDL> str

<?xml version="1.0"encoding="UTF-8" standalone="no" ?><TOP>




<theta ARRAY="4 2"TYPE="4">23 334 4 41 1 2 3 4</theta>


Then, to turn the resulting string back into an OrderedHash, use:

pro hash::FromXml_,_ref_extra=extra

 compile_opt idl2,logical_predicate, static

 oXml = IDLffXmlDomDocument()

 oXml.Load, _strict_extra=extra

 top = oXml.GetDocumentElement()

 nodes = top.GetChildNodes()

 for i=0, nodes.GetLength()-1 do begin

   n = nodes.Item(i)

   if isa(n, 'IDLffXmlDomElement') then begin

     array = []

     key = n.GetTagName()

     txt = n.GetFirstChild()

     if isa(txt, 'IDLffXmlDomText') then str = txt.GetData()

     type = n.GetAttribute('TYPE')

     array = n.GetAttribute('ARRAY')

     if type eq 7 && ~keyword_set(array) then val = str $

     else if type eq 7 then begin

       val = reform(strsplit(str, string(10b), /extract), $

          long64(strsplit(array, /extract)))

     endif else begin

       if keyword_set(array) then begin

          val = make_array(long64(strsplit(array, /extract)),type=type)

       endif else val = (make_array(1, type=type))[0]

       reads, str, val


     self[key] = val




Using a static method helps the syntax:

function OrderedHash::FromXML,_ref_extra=extra

 compile_opt static

 objref = OrderedHash()

 objref.FromXML_, _strict_extra=extra

 return, objref


Here is an example converting the string variable "str" back into an OrderedHash:

IDL> newparam = OrderedHash.FromXML(string=str)

IDL> newparam


   "sigma": 0.78539816339744828,

   "xsize": 512,

   "name": "Exelis",

   "theta": [[23.000000, 334.00000, 4.0000000, 41.000000],[1.0000000, 2.0000000, 3.0000000, 4.0000000]]


This small example does not support nested hash objects, lists, and has a problem with string arrays containing newline characters.

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